This invention relates to methods and materials for the preparation of silicon nitride based ceramic articles. More particularly, it is concerned with powder mixtures and compacts for pressureless sintering, and with methods for preparing such powder mixtures and compacts and with methods for preparing fully dense silicon nitride ceramic articles therefrom.
Densified composite ceramic articles based upon silicon nitride have been the subject of considerable recent study. Silicon nitride powder mixtures when consolidated to near theoretical density possess desirable high temperature properties. These ceramic materials have refractory character, high temperature creep resistance, thermal shock resistance, and high temperature strength which make them ideal for many high temperature application.
Three methods have generally been employed to produce densified ceramic articles from powder mixtures: hot pressing, hot isostatic pressing, and so-called pressureless sintering. Hot pressing involves the simultaneous application of heat and mechanical pressure to the powder mixture at temperatures high enough to cause sintering. Typically, pressures of several tons per square inch are applied to the powder mixture, usually by means of a uniaxial ram press.
In the hot isostatic pressing method, the powder mixture is placed in a non-permeable deformable container to which heat and pressure are applied. In this method, pressure is applied equally to all faces of the powder compact, usually by means of a pressurized fluid.
So-called pressureless sintering of powder compacts generally connotes the process of thermally densifying pre-pressed powder compacts without the use of a container for the compact and without the aid of high pressures applied externally to the compact to drive the powder particles together.
Sintering is the preferred method of densifying composite ceramic articles because of the cost and complexity of equipment needed for other methods of densification.
The fabrication of silicon nitride ceramics from powder mixtures has generally been successful only when densifying additives are added which allow for liquid phase sintering to occur. Even then, the ease of pressureless sintering of silicon nitride varies markedly with the particular densifying additive employed. High temperatures can often cause the undesired thermal decomposition or volatilization of components of the powder mixture. To counteract this problem, it has been the practice to add small amounts of lower melting oxides such as aluminum oxide, magnesium oxide or the like to mixtures of silicon nitride and yttrium oxide to enhance the sintering process by lowering the temperature required for liquid phase sintering. However, addition of such lower melting oxides often has the undesirable effect of reducing the high temperature strength, creep resistance, and oxidation resistance of the final densified ceramic.